1) Field of the Invention
The present invention relates to a fixing member that employs an electro magnetic induction heating to fix an image to a recording member.
2) Description of the Related Art
An image forming apparatus that employs a toner in order to form a visible image is provided with a fixing device to fix a toner image on a recording material. Different approaches are used to perform the fixing: apply heat and/or pressure to the toner image, use a solvent and the like. However, application of heat or pressure has been the most popular method. A fixing device that applies heat (hereinafter, “heat fixing method”) includes a fixing roller and a pressure member that is in pressure contact with the fixing roller. The fixing roller is heated and then the recording material that bears the toner image is passed between the fixing roller and the pressure member. As a result, the toner image is fixed on the recording material. In a variation, the toner image is transferred from an intermediate transfer drum on to a fixing member and then the toner image on the fixing member is fixed to the toner image on the recording material.
However, in the heat fixing method, the conversion of energy is inefficient, the power consumption is high, and the warming up time is long. To overcome these problems, eddy currents heating method and induction heating method came to be used. In the eddy currents heating method the fixing roller is heated by the eddy currents due to the electromagnetic waves, while in the induction heating method, a fixing roller that is formed from a metal conductor is heated by energizing a heating element. These direct heating methods proved highly efficient; because, the heating time is very less. These heating methods also gained popularity since the rigidity of the fixing roller was not affected by the heat.
Many patents related to the direct heating methods have been disclosed, some of which are mentioned below. See, for example, Japanese Patent Laid-Open Publication No. 2000-242108. In the technology disclosed in this publication, a heating belt is placed opposite to an electromagnetic induction heating device. The heating belt includes a heat-resistant resin or rubber base on which is disposed an organic conductive layer having the property of heating by electromagnetic induction and which gets heated by an eddy current loss. Alternatively, the heating belt may include a non-woven fiber base and on which is disposed a conductive layer having the property of being heated by electromagnetic induction and which gets heated by an eddy current loss.
Japanese Patent Laid-Open Publication No. 2002-334774 also discloses a related technology. In the technology disclosed in this publication, a fixing member is equipped with a high frequency source HFS that outputs high frequency of over 100 kHz, an induction coil IC biased by the high frequency output of the high frequency source HFS, and a fixing roller HR. The fixing roller HR has a conductive layer of a thickness less than that of the skin, and the conductive layer gets heated by the secondary current flowing in the induction coil IC in the circumferential direction.
In the fixing member employing direct heating method, it is required that the layers of the fixing member are in tight contact. Otherwise, heat conduction is not uniform and efficient and problems such as unevenness of temperature, reduced resistance to pressure, etc. may arise. Particularly, in a high-temperature and high-moisture environment (for instance a temperature of 30 degrees and humidity of 70%) such as during the rainy season of the inventor's country, if moisture absorption in a layer exceeds 0.07 mg/cm2, during heating a sharp expansion takes place due to evaporation and as a result the conductive layer peels off.
A solution for such a problem is provided in Japanese Patent Laid-Open Publication No. 2001-201966. This publication discloses a heat generating roller in which is provided a resistive element through an insulation on the inner perimeter of the base material. This forms a bump on the inner perimeter of the base material. The concave portion and the opening in the insulation layer for ventilation open to the outside. However, since the heat producing part is separated from the surface layer, the rise time is somewhat longish for a direct heating method.
In recent times, the popularity of color image forming apparatuses is on the increase. There is a demand for realizing high-quality pictures by realizing a fixing member with a surface which is flexible enough to allow toners of multiple colors to be uniformly transferred to a recording material by pressure.
However, in the structure described in Japanese Patent Laid-Open Publication No. 2001-201966, the inner perimeter of the metal base has plural layers such as heat-producing layer (conductive layer), etc. This adversely affects the flexibility of the surface of the heating roller. Further, for the regular high frequency of 20 kilohertz (kHz) to 100 kHz used in the electromagnetic induction in the structure disclosed in patent literature 1, the thickness organic conduction layer (skin thickness) should preferably be 1 micrometer (μm) to 50 μm. The conductive layer which has a thickness of this range is bound to have poor flexibility.